Patent GB-907895-A (GB)
Title: Improvements relating to apparatus for indicating small movements
Inventor: DYSON JAMES
IPC: G01D5/18
Filed: 1959-11-17
Abstract: 907,895. Indicating small movements. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Nov. 16, 1960 [Nov. 17, 1959], No. 38980/59. Class 40 (3). Apparatus for indicating small lateral movements of a body from a straight line reference axis comprises an optical object attached to the body and adapted to emit two beams of light having the same frequency and equally modulated in amplitude, but differing in phase, an optical system adapted to cause said beams of light to tend to form an image of said object coincident with said object for all positions of said object on the optical axis of the optical system, and a light-responsive device arranged to receive light from said image, and the arrangement is such that when the body is on the reference axis the light-reponsive device receives equal amounts of light from said modulated beams of light, and lateral movements of said body from the reference axis cause said light-responsive device to receive unequal amounts of light from each of said modulated beams and thereby produce a phase change in the output of the light-responsive device, and means is included to detect and indicate the variation in the phase of the variable component of the total light received by the light-responsive device and hence the movements of the body from the reference axis. As described in Specification 870,461, an enclosure 9 movable along the common optical axis X, Y of mirrors 1, 2 and incorporating semi-transparaent mirrors 5, 6 forms an image 8 invariably located with respect to an object 7 in association with the identical curved mirrors 1, 2 if the distance of the object 7 from the axis X, Y remains constant. In the arrangement shown in Fig. 2, light sources 14, 15 mounted in a container 11 provided with a diffusing screen 13 and separated by an opaque screen 12, are energized by A.C. sources of equal amplitude and frequency, but out of phase by 45 degrees, so that the light outputs are in phase quadrature. The image is formed at a slit 17 in an opaque member 16 behind which are located a diffusing screen 18 and a photo-cell 19. Normally the luminous image is disposed centrally of the slit 17 so that the equal amounts of light at a different phase produce from the cell 19 an output at a predetermined phase. If the position of the enclosure 9 is altered relative to the axis X, Y the amounts of light incident on the cell 19 at the two different phases are no longer equal and produce a change of phase in its output which can be passed to a phase-detecting means to provide indication of the lateral movement of the body. The A.C. sources for the lamps 14, 15 may be 90 degrees out of phase so that the light outputs are in phase opposition, and the cell 19 has a zero output when the enclosure 9 is located at the correct distance from the axis X, Y. In a modification, Fig. 3 (not shown), the modulated light is provided by two constant light-sources in conjunction with two Kerr cells to the respective pairs of electrodes of which are applied operating voltages equal in amplitudes and frequencies, but having the desired difference in phase. The arrangements may be used to control the movement of a lathe tool relative to a fixed lathe bed by effecting appropriate movement of the tool member or operating a servo arrangement to restore the tool to the on-line position.